Field of the Invention
The present invention relates in general to brake systems and in particular to an exhaust brake system incorporating a slide type valve.
Generally, it is known that an exhaust brake system is adapted to render a load upon a prime mover or an engine by closing a valve incorporated in the exhaust gas passage of the engine, thus producing a braking effect to be relayed upon the driving wheels of a vehicle, and such an exhaust brake system is employed mainly in a large-sized vehicle such as a motor truck or a motorbus. For this type of exhaust brake system, there are generally two types of mechanism employed, that is, a slide valve type and a rotary valve type. In an exhaust brake of slide valve type, it is inevitable that a substantial resistance may be rendered upon the sliding motion of the valve elements when used under a large exhaust pressure, which may possibly make the action of a valve element involved not smooth particularly in its returning motion.
In this respect an arrangement has been suggested as shown in FIG. 2, including an exhaust brake system such that there are two return springs 3 employed for a valve piston 2 for driving a closing valve 1. In FIG. 2, there are shown an exhaust pipe designated at the reference numeral 4, a valve cylinder at 5, a pressure chamber at 6, an atmospheric pressure chamber at 7 and a piston rod at 8. In this exhaust brake system, when compressed air is introduced into the pressure chamber 6, the valve piston 2 is caused to move along the valve cylinder 5 downwardly as viewed in FIG. 2 against the urging force of the return spring 3, and when the exhaust pipe 4 is closed by the closing valve 1, the exhaust brake is put into operation. And, when compressed air within the pressure chamber 6 is exhausted outside, the valve piston 2 is caused to move upwardly as viewed in FIG. 2 in its return stroke under the urging force of the return spring 3, thereby causing the closing valve 1 to be opened to release the exhaust brake system. When the valve piston 2 is in its return stroke with the closing valve 1 being put under a current exhaust pressure, there is a relatively large resistance against the sliding motion of the closing valve 1, however, the valve piston 2 may turn to move relatively smoothly in its return motion by virtue of the increased effect of urging given from two return springs 3 as provided in this arrangement.
However, with an increased urging force available from the return springs 3 in this arrangement of the exhaust brake system, the period of tthe working or opening effort or the valve piston 2 may possibly become short. In order to compensate for such shortage in the working effort, it would be necessary to have an increased working diameter of the cylinder 5, which is another inconvenience in design.
In coping with such a contradictory requirement, there is proposed such an improvement in the design of the exhaust brake system as typically shown in FIG. 3. This specific construction is contemplated in an attempt to have only the initial returning force of the valve piston 2 increased, in which a valve cylinder 5 may have an average diameter, not in excess from the general design. More specifically, by virtue of such an advantageous arrangement that the valve piston 2 is adapted to bias an auxiliary return spring 9 at the completion of working stroke, the working effort of the valve piston 2 would not become short in practice. On the other hand, however, it is notable that there is a greatest resistance against the sliding motion of the closing valve 1 only at the initial stage of the returning stroke of the valve, which would then lead to a sudden decrease in the pressure differential across the closing valve 1, once it opens to a certain extent, thus resulting in a substantial decrease in the resistance against the sliding motion of the valve element. The arrangement as shown in FIG. 3 is contemplated in consideration of the above, in which the valve piston 2 is pushed along with its returning motion by the auxiliary return spring 9 only at the initial stage of returning motion of the piston 2.
With such an improvement in construction of an exhaust brake system as noted above, it is pointed out that this system has such drawbacks as follows. More specifically, the system shown in FIG. 3 has drawbacks at the end of a working stroke of the valve piston 2 when it comes to abut the auxiliary return spring 9. At this point there is produced the greatest differential pressure across a closing valve 1, thus having an increased resistance against the sliding motion of the piston. For this reason, the closing valve 1 must be shut-off as soon as practicably possible.
It is inevitable, however, that there occurs a substantial decrease in the working effort of the valve piston 2 upon its abutting against the auxiliary return spring 9, which means a possible deley in the completion of a shut-off action of the closing valve 1.